1. Field of the Invention
The present invention relates to a piezoelectric resonator used in, for example, a trap circuit or a discriminator, and more particularly, to an energy-trap type piezoelectric resonator which includes a piezoelectric resonance element and a package having a space for allowing vibration of a vibrating portion of the piezoelectric resonance element.
2. Description of the Related Art
Japanese Patent Publication (kokoku) No. 7-70941 discloses a piezoelectric resonator including an energy trap type piezoelectric resonance element which utilizes a thickness extensional vibration mode. The structure of the piezoelectric resonator will next be described with reference to FIGS. 10 and 11.
FIG. 10 is an exploded perspective view of the piezoelectric resonator. FIG. 11 is a vertical sectional view of the piezoelectric resonator of FIG. 10.
A piezoelectric resonator 51 includes an energy trap type piezoelectric resonance element 52 which utilizes a thickness extensional vibration mode. The piezoelectric resonance element 52 includes a rectangular piezoelectric substrate 52a. A first excitation electrode 52b is disposed on the upper surface of the piezoelectric substrate 52a at the center thereof. A second excitation electrode 52c is disposed on the lower surface of the piezoelectric substrate 52a at the center thereof, so that the excitation electrodes 52b and 52c face each other with the piezoelectric substrate 52a disposed therebetween.
The excitation electrodes 52b and 52c are connected with lead electrodes 52d and 52e, respectively. A lead electrode 52d extends along the upper surface of the piezoelectric substrate 52a to one side edge thereof. The lead electrode 52e extends along the lower surface of the piezoelectric substrate 52a to the side edge of the substrate 52a opposite to the side edge to which the lead electrode 52d extends.
Sealing substrates 53 and 54 are attached onto the upper and lower surfaces, respectively, of the piezoelectric resonance element 52. Each of the sealing substrates 53 and 54 is made of an insulating ceramic, such as alumina, and has a rectangular block shape. A cavity 53a is formed in the lower surface of the sealing substrate 53, and a cavity 54a is formed in the upper surface of the sealing substrate 54. The cavities 53a and 54a are arranged so as to be displaced relative to each other when the sealing substrates 53 and 54 are placed on the piezoelectric resonance element 52.
In the piezoelectric resonator 51, the sealing substrates 53 and 54 are bonded to the piezoelectric substrate 52a via an insulating adhesive layer.
A portion of the piezoelectric resonance element 52 sandwiched between the first and second excitation electrodes 52b and 52c functions as a vibrating portion. As shown in FIG. 11, since the vibrating portion is disposed within the cavities 53a and 54a, the vibrating portion, when excited, can freely vibrate within the space defined by the cavities 53a and 54a.
Reference numerals 55 and 56 denote external electrodes. The external electrode 55 is electrically connected to the lead electrode 52d, and the external electrode 56 is electrically connected to the lead electrode 52e.
In the piezoelectric resonator 51, leakage vibration of the fundamental harmonic excited at the vibrating portion is damped at a bonded portion between the piezoelectric resonance element 52 and the sealing substrates 53 and 54. For example, at a portion where the sealing substrate 53 and the upper surface of the piezoelectric resonance element 52 are bonded via an insulating adhesive layer (not shown), leakage vibration of the fundamental harmonic is damped. Through adjustment of the amount of deviation between the cavities 53a and 54a, the amount of damping of leakage vibration of the fundamental harmonic can be adjusted, thereby obtaining a desired frequency characteristic.
Recently, in an effort to increase the operating frequency using a structure similar to that of the piezoelectric resonator 51, the third harmonic has been utilized. In a piezoelectric resonator which utilizes the third harmonic, the fundamental harmonic and fifth and higher odd harmonics become spurious vibration. Accordingly, unless the fundamental harmonic is reliably damped, utilization of the third harmonic may fail, potentially resulting in anomalous oscillation. Since the gain of an IC tends to decrease with frequency, the fundamental harmonic having the lowest frequency easily satisfies the oscillation conditions, so that the fundamental harmonic is highly likely to cause anomalous oscillation. Therefore, reliable damping of the fundamental harmonic has been highly demanded.
In order to suppress the fundamental harmonic without affecting the third harmonic, damping may be effected at a region outside of the vibrating portion, to which region the fundamental harmonic leaks. The region where the fundamental harmonic leaks move, depending on the diameter of the excitation electrodes 52b and 52c, is in concentric regions with respect to the centers of the excitation electrodes. Accordingly, the sizes of the cavities 53a and 54a need to be modified according to the diameter of the excitation electrodes 52b and 52c.
In a piezoelectric resonator in which a space adapted to ensure vibration of a vibrating portion thereof is formed within a package, the fundamental harmonic may be suppressed by either of the following methods: (1) the size of the space is modified according to the diameter of excitation electrodes in order to ensure the vibration; and (2) as in the case of the piezoelectric resonator 51, the upper and lower cavities 53a and 54a are displaced relative to each other.
However, method (1), in which the size of the space is modified according to the electrode diameter, requires a variety of sealing substrates having cavities of different diameters. Thus, not only are expensive dies required, but also the manufacturing processes become complicated, resulting in an increase in manufacturing cost.
In method (2), in which the cavities 53a and 54a are displaced relative to each other, even when the displacement is established by machining dies or by polishing end surfaces of the sealing substrates, use of a variety of sealing substrates is still involved. Thus, not only are relatively expensive dies involved, but also the manufacturing processes become complicated, resulting in an increase in manufacturing cost.
Further, when the end surfaces of the sealing substrates are polished after formation of the cavities in order to establish the relative displacement between the cavities, the relative displacement cannot be accurately controlled, since the quality and workmanship of the polishing determines the accuracy of the relative displacement.